Vital curiosity.
That need to see the next horizon line, to taste life, to participate in it, to have a mind like a sponge. Without asking questions, without pursuing answers purposefully and diligently, your engine can and will lose steam, putter out, derail, and turn into rust.
Don’t tempt that fate, and never stop asking. Take a cue from the toddlers: they know perfectly well that you never know the real answer to “Why?” until you’ve asked at least five times.